Sunday 14 July 2013 16.15 EDT
First published on Sunday 14 July 2013 16.15 EDT

The police and Ministry of Defence have launched an investigation into the deaths of two Territorial Army soldiers during a training exercise in the Brecon Beacons on the hottest day of the year.

The men, understood to be in their late teens or early 20s, were on a routine exercise in the national park in Powys, mid-Wales, on Saturday, when temperatures reached 30C (86F). A third man is in a serious condition in hospital.

Few details have emerged about what caused the deaths but they appear to be related to the heat and physical exertion. A military source said the exercise the men were taking part in was "intense" and their deaths did not involve firearms or a fall.

With three men affected, questions will inevitably be asked about whether their commanders pushed them too far. TA personnel are unlikely to be as fit as fulltime soldiers. Those responsible for army training and exercises will await a police report before conducting their own postmortem. The police are taking the lead in the investigation into the deaths but defence sources said there did not appear to be anything suspicious about them.

"The MoD can confirm that it is working with Dyfed-Powys police to investigate an incident during a training exercise on the Brecon Beacons on Saturday in which two members of military personnel died," the MoD said in a statement. "The two servicemen's next of kin have been informed. More information will be released in due course but it would be inappropriate to comment further at this stage."

A huge variety of British and overseas service personnel are tested and toughened up in the Brecon Beacons. Some of the most demanding terrain is to be found close to the spot where the two men died, and is the venue for the notorious "Fan Dance", a gruelling trek over Pen y Fan, at 866 metres the highest peak in southern Britain. The men died in an area close to the Storey Arms activity centre, a well-known meeting place for walkers heading up Pen y Fan.

Ken Jones, a former member of the special forces who now organises an endurance race in the Brecon Beacons, said service personnel who trained there were sometimes pushed to breaking point.

"They are out there carrying a third or half of their body weight. When it's hot it's impossible to get rehydrated. They have no time for recovery. They are working as hard as Olympic athletes but without the good food, the time to recover, the massage." Jones said he believed both SAS and SAS reservists were undergoing selection in the hills at the moment.

David Davies, the Conservative MP for Monmouth and a former member of the TA, told BBC News: "It's a terrible tragedy, and I'm sure the army will hold a full investigation and be doing absolutely everything to find out what went wrong, and prevent that from happening again."

Davies added: "Nobody should jump to any conclusions here about what may have caused this, but obviously we all know that, for example, people who take part in marathons run a small risk of dying of heat stroke and all sorts of other things, and the British Army does train its soldiers very very hard indeed. The training is there for a purpose, and it is arduous training at any level, and sometimes things tragically go wrong."

Davies said training could involve carrying very heavy equipment and working to a deadline, which meant soldiers would have to push themselves hard in high temperatures.

"It probably would've been cooler on the Brecon Beacons yesterday than it is in the desert of Afghanistan."

Major Alan Davies, who was involved in contingency planning during the first Gulf war, said the Beacons were challenging terrain. "On one end of the spectrum you have cadets being taken for mountain walking and at the other end of the spectrum the SAS use it," he told the BBC.

The incident may have wider ramifications for the government's plans to beef up the TA's role. It is due to be renamed the Army Reserves and its numbers increased by 50% to 30,000. The plan is that the reserves will be more integrated and play a bigger part in military operations as the regular army faces heavy cuts.

News of the deaths was met with shock in the nearby town of Brecon, which is home to the Infantry Battle School (IBS). The rough and remote terrain of the Brecon Beacons is used by the military for a wide range of exercises for various army personnel. The IBS is based at Brecon and regular and special forces soldiers are regularly tested and trained on the unforgiving landscape. Infantry regiments train at Sennybridge in the area, there is an army base in Brecon and the SAS has a base at nearby Hereford.

Matthew Dorrance, the mayor of Brecon, said: "This is incredibly sad. Our thoughts are with the friends and families of those that have lost their lives and with the person who has been injured. Everyone knows someone who works in the army. It's a major employer here and we're very proud of our military links, which go back many years."

Earlier this year an army captain, Rob Carnegie, was found dead on the snow-covered Corn Du mountain in the park. It was thought he was attempting a gruelling march in freezing conditions in a selection process for the SAS.

The tragedy brings back the memory of Private Gavin Williams' death in 2006. He suffered heatstroke in July 2006 after collapsing at a barracks in Wiltshire. Three fellow soldiers were later cleared of his manslaughter after the prosecution claimed they had "beasted" him – subjected him to intense physical exercise. Williams was from south Wales and a member of the 2nd Battalion the Royal Welsh Regiment.